U.S. patent number 5,365,922 [Application Number 07/671,586] was granted by the patent office on 1994-11-22 for closed-loop non-invasive oxygen saturation control system.
This patent grant is currently assigned to Brigham and Women's Hospital, Inc.. Invention is credited to Daniel B. Raemer.
United States Patent |
5,365,922 |
Raemer |
November 22, 1994 |
Closed-loop non-invasive oxygen saturation control system
Abstract
An adaptive controller for delivering a fractional amount of
oxygen to a patient. The controller utilizes an oximeter coupled by
a non-invasive sensor to the patient for measuring the blood
hemoglobin saturation in the patient. The oximeter generates a
plurality of blood saturation output signals over a given period of
time which are sequentially representative of the patient's blood
hemoglobin saturation. A processing means evaluates a plurality of
the oximeter output signals and, based on the evaluation, provides
a pseudo blood saturation signal. A feedback control means
responsive to the pseudo output signal sets the fractional amount
of oxygen to be delivered to the patient. When deviations of the
oximeter output signal are excessive, the pseudo output signals
cause a gradual increase in the fractional amount of oxygen for the
patient. Furthermore, the feedback control means is periodically
disconnected, and the response of the patient to random changes in
the fractional amount of oxygen delivered to the patient is used to
adapt the response characteristics of the feedback control means in
a manner tailored to the needs of the patient.
Inventors: |
Raemer; Daniel B. (Brookline,
MA) |
Assignee: |
Brigham and Women's Hospital,
Inc. (Boston, MA)
|
Family
ID: |
24695123 |
Appl.
No.: |
07/671,586 |
Filed: |
March 19, 1991 |
Current U.S.
Class: |
128/204.23;
128/202.22; 128/204.21; 128/205.11; 128/205.23 |
Current CPC
Class: |
A61B
5/0833 (20130101); A61B 5/097 (20130101); A61M
16/026 (20170801); A61M 16/0051 (20130101); A61B
5/14551 (20130101); A61M 2230/205 (20130101) |
Current International
Class: |
A61B
5/00 (20060101); A61B 5/083 (20060101); A61B
5/08 (20060101); A61B 5/097 (20060101); A61M
16/00 (20060101); A61M 016/00 () |
Field of
Search: |
;128/204.18,204.21,204.23,716,719,202.22,205.23,205.11 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Clement Yu, et al., "Improvement in Arterial Oxygen Control Using
Multiple-Model Adaptive Control Procedures," IEEE Transactions on
Biomedical Engineering, vol. BME-34, No. 8, Aug. 1987, pp. 567-574.
.
Adaptive Control of Arterial Oxygen Pressure of Newborn Infants
Under Incubator Oxygen Treatments, Sano, A., et al., IEE
Proceedings, vol. 132, Pt.D., No. 5, Sep. 1985. .
"Dynamic System Identification: Experiment, Design and Data
Analysis" by Graham C. Goodwin and Robert L. Payne, Academic Press,
New York, 1977. .
"Digital Parameter-Adaptive Control of Processes with Unknown Dead
Time" by Kurz et al., Automatica, vol. 17, No. 1, pp. 245-252,
1981, Permagon Press. .
"The Self-Tuning Controller: Comparison with Human Performance in
the Control of Arterial Pressure", Stern et al., Annals of
Biomedical Engineering..
|
Primary Examiner: Burr; Edgar S.
Assistant Examiner: Lewis; Aaron J.
Attorney, Agent or Firm: Wolf, Greenfield & Sacks
Claims
What I claim is:
1. An adaptive controller for delivering a fractional amount of
oxygen to a patient, said controller comprising:
oximeter means coupled by a non-invasive sensor to said patient for
measuring blood hemoglobin saturation in the patient, said oximeter
generating a plurality of blood hemoglobin saturation output signal
values over a given period of time, sequentially representative of
said blood hemoglobin saturation;
means for generating a running average of said blood hemoglobin
saturation output values;
means for generating pseudo-output signals that are a function of
said running average;
processing means including means for identifying possibly invalid
output signal values and being responsive to said blood hemoglobin
saturation output signal values for evaluating a plurality of said
blood hemoglobin saturation output signal values and, based on said
evaluation, providing a processed output signal;
means for substituting respective pseudo-output signals for each of
said possibly invalid output signal values thereby forming a
sequence of valid output signals.
2. The adaptive controller of claim 1 wherein said oximeter is a
pulse oximeter and said plurality of blood hemoglobin saturation
output signal values are oxygen saturation of hemoglobin values as
measured by a pulse oximeter (S.sub.p O.sub.2 values).
3. The adaptive controller of claim 1 wherein said processing means
includes:
artifact recognition means for identifying possibly invalid output
signal values and for providing a sequence of valid output signal
values, exclusive of said identified possibly invalid output signal
values; and
means for generating a running average of said sequence of valid
output signal values and for providing said running average as said
processed output signal.
4. The adaptive controller of claim 3 wherein said artifact
recognition means includes means for comparing each of said
plurality of output signal values to said running average and for
identifying as possibly invalid any output signal value which
differs from said running average by more than a predetermined
amount.
5. The adaptive controller of claim 3 wherein said means for
generating said running average includes means for calculating a
mean value and a standard deviation value for m of said plurality
of valid output signal values most recently provided by said
artifact recognition means, where m is an integer, and for
providing said calculated mean value as said processed output
signal.
6. The adaptive controller of claim 5 wherein said artifact
recognition means includes means for comparing each of said
plurality of output signal values to said mean value and for
identifying as possibly invalid any output signal value which
differs from said mean value by an amount greater than two time
said standard deviation value.
7. The adaptive controller of claim 5 wherein the feedback control
means includes:
means for generating a difference signal having a value which
represents the difference between the processed output signal and a
value representing a desired blood hemoglobin saturation value;
and
means, responsive to said difference signal, for changing the
fractional amount of oxygen delivered to the patient in a sense
which tends to reduce the value of said difference signal.
8. The adaptive controller of claim 7 wherein the means for
changing the fractional amount of oxygen delivered to the patient
includes a proportional-integral-derivative controller.
9. An adaptive controller for delivering a fractional amount of
oxygen to a patient, said controller comprising:
oximeter means coupled by a non-invasive sensor to said patient for
measuring blood hemoglobin saturation in the patient, said oximeter
generating a plurality of blood saturation output signal values
over a given period of time, sequentially representative of said
blood hemoglobin saturation;
processing means responsive to said blood saturation output signal
values for evaluating a plurality of said output signal values and,
based on said evaluation, providing a processed output signal;
and
feedback control means continuously responsive over a full range of
patient activity to said processed output signal for determining
the fractional amount of oxygen to be delivered to the patient;
wherein the processing means includes:
artifact recognition means for identifying possibly invalid output
signals values and for providing a sequence of valid output signal
values, exclusive of said identified possibly invalid output signal
values; and
means for generating a running average of valid output signal
values and for providing said running average as said processed
output signal; and
wherein the artifact recognition means further includes means for
substituting respective pseudo output signal values for each of
said identified possibly invalid output signal values to generate
said sequence of valid output signal values, wherein the value of
said pseudo output signal values is a function of said running
average of said oximeter means output signal values.
10. The adaptive controller of claim 9 wherein said processing
means further includes:
monitoring means, coupled to said artifact recognition means, for
determining the frequency of occurrence of said possibly invalid
output signal values; and
means, coupled to said monitoring means, for decreasing the value
of said pseudo output signal values in proportion to the frequency
of occurrence of said possibly invalid output signal values to
condition the feedback control means to gradually increase the
fractional amount of oxygen to be delivered to the patient.
11. An adaptive controller for delivering a fractional amount of
oxygen to a patient, said controller comprising:
measuring means, coupled to the patient, for measuring blood oxygen
level in the patient and for providing a plurality of output signal
values, sequentially representative of said measured blood oxygen
level;
processing means, coupled to said measuring means, for generating a
running average of said plurality of output signal values and for
subtracting said running average from a target value, representing
a desired blood oxygen level for the patient, to produce a
difference signal;
feedback control means, coupled to receive said difference signal,
for adjusting the fractional amount of oxygen to be delivered to
the patient to minimize said difference signal in magnitude.
12. An adaptive controller for delivering a fractional amount of
oxygen to a patient, said controller comprising:
measuring means, adapted to be coupled to the patient, for
measuring blood oxygen level in the patient and for providing a
plurality of output signal values, sequentially representative of
said measured blood oxygen level;
processing means, coupled to said measuring means, for generating a
running average of said plurality of output signal values and for
subtracting said running average from a target value, representing
a desired blood oxygen level for the patient, to produce a
difference signal;
feedback control means, coupled to receive said difference signal,
for adjusting the fractional amount of oxygen to be delivered to
the patient to minimize said difference signal in magnitude;
wherein the feedback controller is defined by a transfer function,
having a plurality of adjustable coefficients, which establishes
how the fractional amount of oxygen delivered to the patient is to
be changed, both in time and in magnitude, in response to a change
in the difference signal.
13. The adaptive controller of claim 12, further including:
means for measuring a plurality of physiological parameters related
to respiration to produce a respective plurality of output signals;
and
adjustment means responsive to said plurality of output signals for
adaptively changing the values of said coefficient values to change
the transfer function of said feedback controller.
14. The adaptive controller of claim 13 wherein said physiological
parameters include one or more of minute ventilation, respiratory
rate and tidal volume; and
said adjustment means includes means for automatically adjusting
said coefficient values as a function of said plurality of output
signal.
15. The adaptive controller of claim 12 further including means for
periodically and automatically varying the coefficient values to
change the transfer function of the feedback controller to track
changing needs of the patient.
16. the adaptive controller of claim 15 wherein the means for
periodically and automatically varying the coefficient values
includes:
means for randomly changing the fractional amount of oxygen to be
delivered to the patient within predetermined minimum and maximum
limiting values; and
coefficient adapting means, responsive to the respective measured
blood oxygen level values resulting from the changes in the
fractional amount of oxygen, for adjusting the coefficient values
to match the response of the control system to the response of the
patient.
17. The adaptive controller of claim 16 wherein the coefficient
adapting means includes:
means for recording the random sequence of changes in fractional
amount of oxygen delivered to the patient and the respective
measured blood oxygen levels resulting from the sequence of changes
in the fractional amount of oxygen;
means for calculating, based on the recorded changes in the
fractional amount of oxygen, the recorded blood oxygen levels and
the transfer function of the feedback controller, an expected
present blood oxygen level; and
means for repeatedly changing the coefficient values and
recalculating the expected present blood oxygen level to minimize
any difference in magnitude between the expected present blood
oxygen level and the measured blood oxygen level currently provided
by the measuring means.
18. The adaptive controller of claim 17 wherein the means for
repeatedly changing the coefficient values includes means for
evaluating the recorded changes in the fractional amount of oxygen,
the recorded blood oxygen levels and the transfer function of the
feedback controller according to a recursive least squares
optimization method.
19. The adaptive controller of claim 12 wherein the feedback
controller is a proportional-integral-derivative (PID)
controller.
20. The adaptive controller of claim 19 wherein the transfer
function, Y(nT)/E(nT), defining the feedback controller is given by
an equation:
where K.sub.3, K.sub.2 and K.sub.1 are respective proportional,
integral and derivative coefficient values, Z is the Z-transform
operator, T is a sampling interval in seconds, n is the number of
current sample values, Y(nT) is the current output value and E(nT)
is the current value of the difference signal.
21. In a system for automatically providing a controlled fractional
amount of oxygen to a patient, including an oximeter for measuring
blood oxygen levels of the patient and a feedback controller,
coupled to the oximeter for adjusting the fractional amount of
oxygen delivered to the patient to maintain the measured blood
oxygen levels within predetermined limits, a safety subsystem to
prevent the system from erroneously administering a hypoxic
mixture, said safety subsystem comprising:
means for measuring the fractional amount of oxygen being delivered
to the patient and for providing a signal indicative thereof
including means for sensing the oxygen level of the air being
provided to the patient using a plurality of sensors;
means for correlating the measured fractional amount of oxygen to a
preferred fractional amount of oxygen as determined by the feedback
control system and for providing an alarm signal when the measured
and preferred fractional amounts of oxygen are determined to be
uncorrelated including means for correlating the preferred
fractional amount of oxygen to each of the plurality of sensed
oxygen levels and for generating said alarm signal if any of the
sensed oxygen levels is not correlated to said preferred fractional
amount of oxygen; and
means, responsive to said alarm signal, for adjusting the
fractional amount of oxygen to be delivered to the patient to a
maximum value said safety subsystem further including means for
monitoring the blood oxygen level signals provided by the oximeter
including means to identify possibly invalid signal values and
means to provide a sequence of valid signal values, exclusive of
said identified possibly invalid signal values; and said safety
subsystem includes means for setting a first threshold value
representing a minimum desirable level of said blood oxygen level
signal and a second threshold value representing a minimum
desirable level of said preferred fractional amount of oxygen and
said measured fractional amount of oxygen; and means for producing
said alarm signal when one of said sequence of valid signal values
is less than said first threshold value; means for producing said
alarm signal when one of said preferred fractional amount of oxygen
and said measured fractional amount of oxygen is less than said
second threshold value.
22. the safety subsystem of claim 21, further comprising:
first monitoring means for detecting errors in the blood oxygen
level signal provided by said oximeter;
second monitoring means for detecting errors in the signal provided
by said means for measuring the fractional amount of oxygen being
delivered to the patient; and
means for generating said alarm signal in response to an error
being detected by one of said first and second monitoring
means.
23. the safety subsystem of claim 22 wherein said first and second
monitoring means detect missing values of said respective blood
oxygen level signal and said measured fractional amount of oxygen
signal as the errors in the respective signals.
24. The safety subsystem of claim 21 further comprising,
non-volatile memory means for storing the measured blood oxygen
level values provided by said oximeter and the measured fractional
amount of oxygen being delivered to the patient.
25. The safety subsystem of claim 24 wherein values representing
said preferred fractional amount of oxygen are stored in said
non-volatile memory means.
26. A method for adaptively controlling the fractional amount of
oxygen delivered to a patient comprising the steps of:
a) measuring the blood hemoglobin saturation in the patient during
a plurality of intervals over a given period of time and providing
said measured values as an output signal;
b) evaluating each of the measured values of said output signal to
identify possibly invalid output signal values;
c) eliminating said identified possibly invalid output signal
values from said output signal to produce a processed output
signal;
d) adjusting the fractional amount of oxygen delivered to the
patient in a sense to minimize any difference between said
processed output signal and a predetermined desired blood
hemoglobin saturation signal.
27. A method for adaptively controlling the fractional amount of
oxygen delivered to a patient comprising the steps of:
a) measuring the blood hemoglobin saturation in the patient during
a plurality of intervals over a given period of time and providing
said measured values as an output signal;
b) evaluating each of the measured values of said output signal to
identify possibly invalid output signal values;
c) eliminating said identified possibly invalid output signal
values from said output signal to produce a processed output signal
by
c1) generating a sequence of valid output signal values by
substituting a pseudo value for each of said identified possibly
invalid output signal values;
c2) generating a running average of successive values of said
sequence of valid output signal values, wherein the instantaneous
value of said running average is said pseudo value;
c3) providing said running average as said processed output signal;
and
d) adjusting the fractional amount of oxygen delivered to the
patient in a sense to minimize any difference between said
processed output signal and a predetermined desired blood
hemoglobin saturation signal.
28. The method of claim 27 wherein step c1) includes the steps
of:
monitoring the frequency of occurrence of said identified possibly
invalid output signal values; and
decreasing the value of said substituted pseudo values in
proportion to the frequency of occurrence of said identified
possibly invalid output signal values.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to automatic control of blood oxygen
saturation (S.sub.a O.sub.2) in a patient by adjustment of the
fractional amount of oxygen inspired (FIO.sub.2) by the patient,
and more particularly, to a feedback control loop for a mechanical
ventilator including a non-invasive S.sub.a O.sub.2 monitor in the
feedback path for developing an adaptive control signal which
controls the inspired gas blender of the ventilator.
2. Description of the Prior Art
Devices for controlling the oxygen content of the blood by
controlling the fraction of oxygen breathed by a patient are well
known. For example, U.S. Pat. No. 2,414,747 issued to Harry M.
Kirschbaum on Jan. 21, 1947 shows a method and apparatus for
controlling the oxygen content of the blood of living animals which
discloses control of blood oxygen content by the use of an ear
oximeter which produces a signal used to control the proportion of
inspired oxygen. As the oximeters direct a beam of light through a
capillary bed in the ear, the characteristics of the light become
modified by the color of the blood that intercepts its path. Thus,
the change in oxygen levels of the blood are detected
non-invasively and electrical signals are generated, amplified and
used to control the oxygen supply delivered to a patient.
Numerous improvements have been made since that time wherein better
matching of oxygen delivery to the needs of the patient have been
made such as shown in U.S. Pat. No. 3,734,091 to Ronald H. Taplin
issued on May 22, 1973. Taplin discloses an optical oximeter and a
temporary oxygen-deficient mixture (anoxic) to control blood oxygen
saturation. Thus, to prevent excessive oxygen levels, Taplin
discloses limiting the inspired oxygen by intermittently providing
the anoxic mixture each time the oxygen saturation of the blood
reaches a predetermined percentage level.
U.S. Pat. No. 4,889,116 issued to Taube on Dec. 26, 1989 discloses
one type of adaptive controller for adjusting the fraction of
oxygen inspired by a patient. The controller utilizes a pulse
oximeter connected by an optical sensor to the patient for
measuring the patient's blood hemoglobin saturation (S.sub.p
O.sub.2) and pulse rate. These signals from the oximeter are used
by a calculator for determining the fractional amount of oxygen to
be inspired by the patient. The calculated percentage of oxygen is
provided to the patient so that the gas taken in by the patient
automatically causes the blood in the patient to reach a
predetermined hemoglobin saturation level in response to the
patient's requirements as determined by changes in the S.sub.p
O.sub.2 signal. However, the calculator is programmed to determine
when there is an excess deviation of the patient's pulse rate,
thereby indicating patient movement and the probability that the
pulse oximeter will provide false S.sub.p O.sub.2 values during
such patient movement. When an excess deviation in pulse rate is
detected, the fractional amount of inspired oxygen is no longer
responsive to the measured S.sub.p O.sub.2 value, but instead held
constant until the excess deviation of the pulse rate has been
terminated. Furthermore, a low S.sub.p O.sub.2 value, indicative of
a depressed respiration (apnea) is also detected, and used to cause
a preset higher percentage of inspired oxygen to be supplied to the
patient until the depressed respiration of the patient has been
terminated. Thus, responsive FIO.sub.2 adjustment is suspended
during patient movement and apnea, and during this time fixed
FIO.sub.2 levels are set.
In 1980, H. Katsuya and Y. Sakanashi published an article in the
Journal of Clinical Monitoring 1989; 5:82-86 describing a method
for evaluating pulmonary gas exchange using a pulse oximeter. They
developed the concept of FI.sub.9x (where x is a single digit
number) which is the fraction of inspired oxygen necessary to
achieve a measured blood oxygen saturation equal to the value of 9x
% (e.g. 98%). This experiment was carried out by periodically
manually increasing or decreasing the FIO.sub.2 control of a gas
blender portion of a ventilator until the S.sub.p O.sub.2
measurement reached the target percentage (e.g., 98%). The purpose
of this experiment was to develop a diagnostic method to evaluate
pulmonary gas exchange impairment. A high value of FI.sub.9x was
associated with poor pulmonary gas exchange. In this publication,
no mention was made of feedback of S.sub.p O.sub.2 values for
automatic adjustment of FIO.sub.2. The present invention recognizes
that the pulmonary impairment of a patient can often change during
treatment, thereby requiring a change or adaptation of the
responsiveness of the FIO.sub.2 control loop.
Since adult and neonatal patients in intensive care units suffering
from respiratory distress are at risk for developing hypoxemia or
oxygen toxicity, certain safety precautions should be taken to
prevent O.sub.2 under/overdose. In an attempt to maintain organ
normoxia, appropriate clinical care often mandates ventilation with
high FIO.sub.2, sometimes for several days. Long exposure to high
concentrations of oxygen has been implicated in complications
including exacerbation of respiratory distress and various central
nervous system symptoms. In neonatal patients, oxygen toxicity may
result in blindness from retrolentalfibroplasia. Thus, care should
be taken to minimize the FIO.sub.2 exposure while maintaining
adequate S.sub.p O.sub.2, so that the onset of these insidious
complications can be delayed or avoided. Furthermore, artifacts
(false output measurements) are commonly found in the pulse
oximeter output due to patient movement and/or low blood perfusion
in the area where the patient contacts the pulse oximeter sensor.
Additionally, it is difficult to actually know what the arterial
blood oxygen saturation percentage is from the S.sub.p O.sub.2
(pulse oximeter) measurement. Thus, careful construction of the
S.sub.p O.sub.2 feedback control system is required.
It is an object of the present invention to provide a method and
apparatus which minimizes the FIO.sub.2 of a patient while
maintaining adequate S.sub.a O.sub.2 levels.
It is a further object of the invention to provide artifact
rejection processing of the pulse oximeter which is tolerant of the
expected false readings of S.sub.p O.sub.2, and furthermore, which
is adaptive so as to gradually cause the FIO.sub.2 to increase as
the frequency of the artifacts increases.
It is still a further object of the invention to provide an
FIO.sub.2 feedback control loop which has a response (transfer
characteristic) which is adaptive to the changing requirements of
the patient.
It is an even further object of the invention to provide a safety
sub-system for the FIO.sub.2 control system in order to prevent a
failure in the feedback control system from causing injury to the
patient due to extremely inappropriate levels of FIO.sub.2.
SUMMARY OF THE INVENTION
In accordance with the principles of the present invention, a
method and apparatus is provided for evaluating the oximeter
S.sub.p O.sub.2 measurements for artifact rejection purposes, and
furthermore, causing a gradual increase in FIO.sub.2 as the
frequency of S.sub.p O.sub.2 artifacts increases.
In accordance with a further feature of the invention, a safety
sub-system is provided which monitors the output signals from
various portions of the FIO.sub.2 control system for correlation
and/or excursion beyond preset threshold values. Signals which
exceed the thresholds or which do not correlate, cause safety
sub-system to indicate an alarm condition to the physician.
In accordance with further features of the invention, a feedback
control means of the FIO.sub.2 control loop has an adaptively
adjustable response. In the preferred embodiment, the feedback
control means comprises a proportional, integral, derivative
controller wherein the coefficients of the P, I, D, terms are
selectively adjustable in order to provide appropriate respiration
therapy to the patient. Additionally, these coefficients may be
modified by input signals which are not part of the FIO.sub.2
control loop, such as minute ventilation, tidal volume, etc.
Finally, the controller response can be adaptive to the needs of
the patient, by periodically disabling the controller and
monitoring the response of the patient to random fluctuations of
FIO.sub.2. An evaluation of the response of the patient is used to
adapt the response characteristic of the feedback controller.
Other features and advantages of the invention will be apparent
from the description of the preferred embodiment and from the
claims.
For a fuller understanding of the present invention, reference
should now be made to the following detailed description of the
preferred embodiment of the invention and to the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a schematic representation of a closed-loop
S.sub.p O.sub.2 control system constructed in accordance with the
present invention for controlling the fractional amount of the
inspired oxygen for a patient;
FIG. 2 is a block diagram of the flow control algorithm used for
artifact rejection in the output signal from the pulse oximeter
shown in FIG. 1;
FIG. 3 is a block diagram of a digital PID controller used in the
feedback control system of the present invention;
FIG. 4 is a block diagram of the computational process of the PID
controller of FIG. 3.
FIGS. 5(a) to 5(c) are graphical representations of the non-linear
operation of the PID controller of FIG. 3;
FIG. 6(a) illustrates a block diagram of the PID controller
transfer function in the Laplace domain and FIG. 6(b) illustrates a
range of values for K3, a term in the controller transfer function;
and
FIGS. 7(a) and (b) illustrate a block diagram and waveform useful
for understanding the RLS method.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 illustrates in functional block diagram form a preferred
embodiment of the present invention. A substantial portion of
functions described below are actually carried out by the control
system computer, which will be described later on with respect to
the user interface. A patient 8 requiring mechanical ventilation is
connected to a mechanical ventilator 10 via a breathing circuit 11.
Breathing circuit 11 includes an inspiration line 12 by which
ventilation gases are unidirectionally delivered to patient 8 and
an expiration line 14 for causing unidirectional flow of expired
gases from patient 8 to mechanical ventilator 10. Ventilator 10 may
comprise any of several well known types of commercially available
mechanical ventilators, such as the Model 900C manufactured by
Siemens Corporation and available from Siemens Medical Systems,
Iselin, N.J. A pulse oximeter 16 of conventional design includes an
optical sensor 18 coupled to the finger of patient 8 for
non-invasively (e.g., optically) providing a measurement S.sub.p
O.sub.2, in a well known manner, of the blood oxygen saturation
level of patient 8. Pulse oximeter 16 may be, for example, of the
type manufactured by Siemens Corporation and comprise the Model 961
monitor system including a pulse oximeter cartridge unit therein.
Pulse oximeter 16 includes an output for providing a high speed
serial digital signal output S.sub.p O.sub.2 representative of the
measured S.sub.a O.sub.2 level.
Before the measured S.sub.p O.sub.2 level can be used for
controlling the FIO.sub.2, certain precautions must be taken. The
present inventor has recognized the importance of providing
artifact rejection filtering techniques to the measured S.sub.p
O.sub.2 level, in order to prevent erroneous operation of the
FIO.sub.2 feedback control system. That is, it is normal during the
operation of a pulse oximeter, that artifact (false) output
information will occasionally be generated in response to e.g.,
physical movement of patient 8 and/or low profusion in the area of
sensor 18. This factor was noted in the forenoted Taube patent,
however, responsive FIO.sub.2 control was suspended during patient
movement. However, in the present invention, an S.sub.p O.sub.2
artifact filter 20 is provided for processing the output of pulse
oximeter 16. In the preferred embodiment, the function of filter 20
is provided completely by software processing in the control system
microprocessor. An algorithm is used which rejects artifactual
measurements of S.sub.p O.sub.2 based on the statistics of past
measurements of S.sub.p O.sub.2. The algorithm replaces measured
values of S.sub.p O.sub. 2 which fall substantially outside the
variance of previous values of S.sub.p O.sub.2 with pseudo values.
These pseudo values are related to the average S.sub.p O.sub.2
values and, in accordance with a further aspect of the present
invention, the frequency of the artifactual S.sub.p O.sub.2 values.
As the frequency of artifacts increases the pseudo values are made
increasingly less than the mean value, thereby causing the
FIO.sub.2 to be gradually increased. Thus, the algorithm provides
an adaptive filtering of the S.sub.p O.sub.2 values and gradual
increase in the FIO.sub.2, based on the frequency of the detected
artifacts. Details of filter 20 will be described later on in
conjunction with FIG. 2.
After appropriate artifact rejection filtering, the S.sub.p O.sub.2
value is then compared with a target S.sub.p O.sub.2 value in a
comparator 21 and the difference signal E(t) (error) is then
applied to a feedback controller module 22. The target S.sub.p
O.sub.2 value is provided by a user interface 24 which may comprise
a personal computer of conventional design programmed in the C
software language and which includes a keyboard, display,
microprocessor and data storage means. The system user will input,
via the keyboard, the desired set-point and limit values for the
blood oxygen saturation levels as well as other patient data. The
user interface display provides data to the physician informing
him/her of the status of the system, the S.sub.p O.sub.2 set-point,
the measured S.sub.p O.sub.2 and the measured FIO.sub.2.
Feedback controller 22 may comprise a PID
(Proportional-Integral-Derivative) controller, such as described in
greater detail later on. Feedback controller 22 receives the error
signal provided by comparator 21 and produces a stable control
signal in response thereto, having a zero steady state error and
limited overshoot. In a preferred embodiment of the invention, the
proportional, integral and/or derivative terms are selectively
controllable for adapting the response of controller 22. Feedback
controller 22 is also adaptive as a function of external input
signals, such as minute ventilation (the amount of gas breathed by
a patient in one minute). Additional novel features of feedback
controller 22 will also be described later on relating to the
periodic introduction of random FIO.sub.2 level changes, which are
used to adapt the response of the feedback compensation. Feedback
controller 22 provides an output which indicates what the FIO.sub.2
level should be, e.g., 30%.
An FIO.sub.2 controller 26, comprised partly of software and partly
of hardware including a stepper motor, calculates the present
set-point of the FIO.sub.2 being delivered to the patient, compares
this with the instructions received from feedback controller 22,
and then mechanically controls a gas blender included in mechanical
ventilator 10 for causing the FIO.sub.2 level to be adjusted. The
gas blender, not specifically shown, is a conventional part added
to or built-in to mechanical ventilator 10 which controls the
mixing of gas from an air source and an O.sub.2 source,
respectively, to the gas input of ventilator 10. For example,
assume that feedback controller 22 provides an output (in software)
to FIO.sub.2 controller 26 demanding that the FIO.sub.2 be
increased to 30%. The software portion of FIO.sub.2 controller 26
will then calculate a number of pulses, e.g., 30 to the right, that
must be applied to the stepper motor in order to cause the gas
blender of mechanical ventilator 10 to cause a 30% mixture between
O.sub.2 and air. This mechanical connection is indicated by the
output from controller 26 to mechanical ventilator 10, although an
electronic control could also be used.
A final portion of the invention comprises safety sub-system 28. A
series of safety features are implemented in hardware and software
which comprise safety sub-system 28 to prevent hypoxic inspired
mixtures and inadvertent errors in the operation of the FIO.sub.2
control system. These include:
1. Limit (threshold) values of S.sub.p O.sub.2 and FIO.sub.2 are
entered into the system by the user via interface 24. Safety
sub-system 28 monitors the artifact corrected S.sub.p O.sub.2 value
via line 29, the FIO.sub.2 output from controller 26 via line 31,
and the actual O.sub.2 level sensed in ventilator 10 and
inspiration line 12 via lines 33 and 35, respectively. Excursions
beyond the pre-set thresholds are detected by sub-system 28 and
cause visual and audible alerts to be directed to the
physician.
2. The system resets (see 4 below) based on missing signals from
pulse oximeter 16 or ventilator 10.
3. The system detects error between desired FIO.sub.2 (i.e., the
output from controller 22 via line 37) and the measured FIO.sub.2
at three different places. The first place is the output of
controller 26 via line 31, the second place is the O.sub.2
indication signal which is provided from mechanical ventilator 10,
via line 33 and the third place is a redundant O.sub.2 indication
signal provided from an O.sub.2 sensor in breathing circuit 11, via
line 35. Each of these values should be correlated; if not, an
alarm condition is indicated.
4. The system automatically sets the output of controllers 22 and
26 to 100% when any of the above signals indicate error, via set
lines S1 and S2 which are coupled to controllers 22 and 26,
respectively, from safety sub-system 28.
5. All data is stored on magnetic media for retrospective
analysis.
Next, details of artifact filter 20 will be described in
conjunction with FIG. 2. This filter algorithm is intended to
handle artifacts in a safe, i.e. conservative, manner. Firstly, it
recognizes that the pulse oximeter is artifact prone because it is
sensitive to both patient movement and blood perfusion in the area
of sensor 18. Secondly, it assumes that when data is missing or
suspiciously different (e.g., low) compared to the date immediately
preceding it, it is probably artifactual. When artifacts are
occurring, the worst scenario is that the S.sub.p O.sub.2 is
actually falling, and increasing the patient FIO.sub.2 would be the
appropriate therapy. Thus, when artifacts are occurring, we want to
balance our concern that the S.sub.p O.sub.2 may be falling with
the knowledge that missing data is a routine occurrence.
Furthermore, as the frequency of missing data increases, our
concern that S.sub.p O.sub.2 might be falling should also increase.
Thus, in accordance with a further aspect of the present invention,
when S.sub.p O.sub.2 data is missing or suspiciously low (e.g.,
more than two standard deviations away from the mean of the
previous values) we will initially assume the true S.sub.p O.sub.2
value is likely to be the mean value. Additionally, in accordance
with still a further aspect of the invention, to be conservative we
will subtract an adaptive factor from the mean value to recognize
the potential that the actual S.sub.p O.sub.2 value may be falling.
The factor is adaptively increased as the frequency of occurrence
of the artifacts increases. This results in progressively
increasing levels of FIO.sub.2 for the patient to be set by the
FIO.sub.2 control system.
The artifact rejection algorithm of filter 20 is designed to insure
that the FIO.sub.2 control system operates normally. As artifacts
occur, the control system will tend to increase FIO.sub.2
gradually. Thus, as the frequency of artifacts increases, the
controller will drive FIO.sub.2 up harder. When the frequency of
occurrence of the artifacts exceeds an intolerable limit, an alarm
will sound and the controller will go open loop with FIO.sub.2
=100%.
As shown in FIG. 2, filter 20 operates in accordance with the
following process:
Step 202: Evaluate the S.sub.p O.sub.2 values for artifacts,
Step 204: Determine the frequency of occurrence for the artifacts,
and
Step 206: Adapt the pseudo S.sub.p O.sub.2 control value in
accordance with the artifact occurrence frequency.
This process is accomplished as follows:
Step 202
As shown in FIG. 1, S.sub.n =current S.sub.p O.sub.2 value from
pulse oximeter 16. S'.sub.n =S.sub.p O.sub.2 value sent to
comparator 21.
S.sub.n-m , . . . , S.sub.n-3 , S.sub.n-2 , S.sub.n-1 , S.sub.n is
a series of m past S.sub.p O.sub.2 output values. ##EQU1##
Then, let M.sub.n =a binary logical variable (possible artifact).
We then set up four tests (windows) to determine if M.sub.n is
TRUE, i.e., an artifact, or FALSE, i.e., probably not an artifact
but a good S.sub.p O.sub.2 value from oximeter 16.
1. If S.sub.n <(S.sub.n -2S.sub.n) and S.sub.n >2 then
M.sub.n =TRUE
2. If S.sub.n <(S.sub.n -4) and S.sub.n .ltoreq.2 then M.sub.n
=TRUE
3. If S.sub.n .gtoreq.(S.sub.n -2S.sub.n) and s.sub.n >2 then
M.sub.n =FALSE
4. If S.sub.n .gtoreq.(S.sub.n -4) and S.sub.n .ltoreq.2 then
M.sub.n =FALSE
As you can see from tests 1 and 3, if the standard deviation is
greater than 2, it is relatively safe to say that if SN has a value
which less than and due two standard deviations below the running
average value, it's probably artifactual, and if S.sub.n is not
less than two standard deviations from the running average, it's
probably valid. For example, if the running average=96% and the
standard deviation is 3%, if the current S.sub.n is less than 90%,
it's assumed to be an artifact and if equal to or greater than 90%,
it is assumed to be valid.
Tests 2 and 4 are intended for the case where the standard
deviation is small, i.e. less than or equal to 2%. Under these
circumstances it would be difficult to use standard deviation as a
measure of validity since even slight changes can be erroneously
considered artifacts. Thus, under these circumstances, a fixed
change in S.sub.n from the running average is used as the
validating criteria. In the preferred embodiment, the fixed amount
is a 4% change. For example, if the running average was 96% and the
current S.sub.n was 93%, it would not be considered an artifact. It
should be understood that the criteria given here are those
currently being used by the inventor, and that future testing may
result in modifications of these values and even the use of other
criteria, such as the average absolute difference of each value
from the mean value.
Step 204
Compute the frequency of artifact occurrences ##EQU2## for M.sub.i
where i=M-m to n-1
Step 206
1. If M.sub.n =FALSE then S'.sub.n =S.sub.n
2. If M.sub.n =TRUE then S'.sub.n =S.sub.n -Q, where Q is one of
q.sub.1, q.sub.2 or q.sub.3.
Where,
Q=q.sub.1 if 1/m<F.sub.n .ltoreq.f.sub.a ;
Q=q.sub.2 if f.sub.a <F.sub.n .ltoreq.f.sub.b ; and
Q=q.sub.3 if f.sub.b <F.sub.n .ltoreq.f.sub.c.
Also:
3. If F.sub.n >f.sub.c then: a) ALARM, b) set FIO.sub.2 =100%;
and c) set S'.sub.n =0.5% (an arbitrary small, non-zero amount)
4. If S.sub.n >1/3 S.sub.n then ALARM
In the preferred embodiment, m=10 sample S.sub.p O.sub.a values,
f.sub.a 3/10, f.sub.b =5/10, f.sub.c =6/10, and q.sub.1 =0.5%,
q.sub.2 =1.0%, and q.sub.3 =2.0%.
Feedback controller 22 of the present invention is of the PID
(Proportional-Integral-Derivative) controller type, and computes a
feedback compensation response which is related to the difference
between the desired (set-point) and measured S.sub.p O.sub.2 values
i.e., the output of comparator 21. The flow of the O.sub.2 gas
which is added to the breathing mixture of the patient is
controlled by an electronic signal from controller 22 which is used
to adjust the stepper motor of controller 26. The electronic signal
is a function of the sum of three terms: ##EQU3## where Y(t) is the
flow control signal, C.sub.1, C.sub.2 and C.sub.3 are constants
E(t) is the Error (difference between the desired S.sub.p O.sub.2
and the measured S.sub.p O.sub.2 (S'.sub.n) and dE(t)/dt is the
time derivative of E(t). The output Y(t) may be adjusted by a
constant of proportionality as circumstance may dictate.
Although the above is an analog implementation of feedback
controller 22, it is also possible to use a sampled-data equivalent
feedback controller, such as commonly used with microcomputers. The
block diagram of a digital PID controller of the preferred
embodiment is shown in FIG. 3. The transfer function of the digital
PID controller is as follows: ##EQU4## The microcomputer of the
user interface system samples the input variable (E(t))periodically
(every T seconds). Computer software is used to mathematically
implement the feedback algorithm of controller 22.
While the output of the digital controller is presented every T
seconds, a circuit element called a zero-order-hold takes the
periodically produced output e(t), holds it steady and implements
the controlling output action.
In the block diagram of the digital PID controller in FIG. 3,
K.sub.3, K.sub.2 and K.sub.1 are coefficients (gain constants) of
the proportional, integral and derivative terms, respectively. T is
the sample time interval, n is an integer number of sample
intervals. Z is the Z-transform operator, E(nT) is the error signal
e(t) sampled every T seconds, and Y(nT) is the desired FIO.sub.2
value Note that K.sub.3, K.sub.2 and K.sub.1 serve the same purpose
as C.sub.1, C.sub.2 and C.sub.3 in the analog implementation
discussed previously; however, the values of K.sub.1, K.sub.2 and
K.sub.3 may be different from C.sub.1, C.sub.2 and C.sub.3.
A block diagram of this process is shown in FIG. 4. The values of
the coefficients as shown in FIG. 4 are dependent on the
characteristics of the design of the system which is being
controlled. The increment (n), error (E), flow (F) and the output
are functions of the volume and flow characteristics of the
breathing circuit, the patient's lung and O.sub.2 transfer
characteristics, and the settings of the ventilator.
It is also possible to use the patient's minute ventilation (MV),
title volume (TV), and/or respiratory rate (RR) to modify one or
more of the terms in the above circuit equation. The patient's
minute ventilation, tidal volume, or respiratory rate may be
measured using any of several well-known techniques. For example,
these measurements may be based on thermal dissipation, a pressure
difference across a resistive element (pneumotachograph), the
rotation rate of a vane, or the oscillation frequency of a fluid
vortex. This type of modification of the invention will be
described next in greater detail with respect to FIG. 6.
Furthermore, in accordance with another aspect of the invention,
design goals such as tolerable overshoot, time to achieve control
and accuracy of control are also used to determine the value of
coefficients K.sub.1, K.sub.2 and K.sub.3. Details relating to
modifications of this type are described in greater detail with
respect to FIG. 7.
First, however, the initial set-up of controller 22 will be
described. Referring to the above equation for the transfer
function of the digital PID controller, the values of K.sub.1,
K.sub.2 or K.sub.3, or any selective combination of K.sub.1,
K.sub.2 and K.sub.3 are adjusted as a function of the error signal
e(t), i.e., the output of comparator 21. The initial set-up of
these values is accomplished in order to tailor the response of the
FIO.sub.2 control system in accordance with the clinical goals for
given classes of patients. For example, there may be a first set of
values for K.sub.1, K.sub.2, K.sub.3 for adults and a second set of
K values for neonatal. patients.
In accordance with a further aspect of the invention, these K
values of the PID controller are made non-linear in order to more
appropriately tailor the response of the FIO.sub.2 system in
accordance with empirically determined desired values for specific
classes of patients. Additionally, it is noted that the non-linear
gain provided by PID controller 22 makes the response of the
FIO.sub.2 control loop more clinically appropriate. For example,
FIG. 5(a) illustrates a value for K.sub.1 as a function of e(t).
However, a limiting value is provided so that the integrator term
does not get to large. Although the previously noted U.S. Pat. No.
4,889,116 to Taube includes a PID controller, it is noted that no
individual control of the proportional or integral or derivative
terms is provided, such as specifically provided by the present
invention.
FIG. 5(b) illustrates an additional non-linear characteristics for
the K values, which was used for the K.sub.3 value specifically in
the preferred embodiment of the present invention and FIG. 5(c) is
an alternative embodiment thereof. Thus, it is apparent from these
figures that non-linear coefficients are provided for various ones
of the terms of the PID controller which change in real time in
accordance with the error signal provided at the output of
comparator 21.
Additionally, it is noted that limiting values are also imposed
upon the output of PID controller, as previously noted, which
prevent the output FIO.sub.2 values from being greater than 100% or
less than 0%.
FIG. 6(a) is illustrative of the way in which external inputs can
be applied to control the coefficients of PID controller 22 in
order to change the system response accordingly. The external
inputs may comprise the minute ventilation (MV), the respiratory
rate (RR) and/or the tidal volume (TV). For example, K.sub.1 could
be a function of minute ventilation or K.sub.1, K.sub.2, K.sub.3
could be functions of RR, MV and TV, respectively. In fact, any
combination of functional relationships between the K's and
external inputs are possible. In the preferred embodiment, the
value of K.sub.3 is changed in response to the minute ventilation
as shown in FIG. 6(b).
A final control of the characteristics for controller 22 relates to
an adaptiveness of its response to the changing needs of the
patient. In this regard, it is noted that in the forenoted Taube
U.S. Pat. No. 4,889,116, the controller program described in
Exhibit A initially asks the user to input a "lung time constant
tl". The present inventor has recognized that the initial values of
k which determine the controller response may not be appropriate
during later periods of the ventilation therapy for the patient. In
accordance with a further feature of the invention, the PID
controller output is periodically and randomly varied (within
prescribed safety limits) in order to adapt the responsiveness of
controller 22 with the changing needs of the patient.
FIG. 7(a) illustrates a functional block diagram of the FIO.sub.2
control loop including a Recursive Least Squares (RLS) computation
used for modifying the response of controller 22. When software
switch A is in position one, the RLS algorithm 700 is dormant.
Under these conditions, the values for the gains of the PID
feedback compensation (K.sub.1, K.sub.2 and K.sub.3) are set at
their initial (default) values.
When RLS algorithm 700 is activated, switch A is in position two,
and a sequence of random magnitude FIO.sub.2 values are applied to
the blender of ventilator 10 via line B. The sequence of random
FIO.sub.2 values are a percentage (.+-.) of a predetermined
FIO.sub.2 value. The predetermined value is the last value of
FIO.sub.2 before the RLS algorithm was started (which is input to
RLS algorithm 700 via line C). The initial value may also be an
average of several previous FIO.sub.2 values in order to eliminate
the possibility that the last FIO.sub.2 value is non-representative
of the actual needs of the patient.
FIG. 7(b) illustrates an example of what one such random sequence
might look like. The random 10 second periods of FIO.sub.2 .+-.m %
occur approximately 20 to 60 times each 15 minutes, in the
preferred embodiment. The range of amplitudes of m is set by
clinical considerations and performance issues. A range of m equal
to 10%, 15% and 20% has been used.
The response of the patient as measured by pulse oximeter 16 is
input to RLS algorithm 700 via line D for processing. When the RLS
algorithm has collected several values, in the preferred embodiment
30, it begins to process these values in accordance with the
Recursive Least Squares (RLS) computation, such as known by those
skilled in the art, and described for example in "Dynamic System
Identification: Experiment, Design and Data Analysis" by Graham C.
Goodwin and Robert L. Payne, Academic Press, New York, 1977; and an
article entitled "Digital Parameter-Adaptive Control of Processes
with Unknown Dead Time" by Kurz et al. in Automatica, Vol. 17, No.
1, pp. 245-252, 1981, published by Permagon Press; and as also
described in conjunction with control of arterial blood pressure in
an article entitled "The Self-Tuning Controller: Comparison with
Human Performance in the Control of Arterial Pressure" by Kenneth
S. Stern et. al., published in the Annals of Biomedical
Engineering.
Briefly, what the RLS algorithm does is find a set of coefficients
(a's and b's) of a linear combination of past and present FIO.sub.2
values and past S.sub.p O.sub.2 values for determining an estimated
S.sub.p O.sub.2 value, which set of coefficients minimizes the mean
square difference between this estimated value of S.sub.p O.sub.2
and the actually measured value of S.sub.p O.sub.2. When values for
a and b are obtained which cause the computed and measured values
to be substantially equal, it can be said that the RLS algorithm
has converged. If 30 samples of S.sub.p O.sub.2 at 10 second
intervals was not enough to cause convergence, the RLS algorithm
will continue acquiring new S.sub.p O.sub.2 values while dropping
the oldest S.sub.p O.sub.2 values and then try to converge. Upon
convergence, new values may be found for the time constant (T) and
time delay (L) of the controller response, which leads to new
values for the K.sub.1, K.sub.2, and K.sub.3, coefficients,
respectively. These new coefficients are coupled to controller 22
via line K.
Thus, there has been shown and described a novel method and
apparatus for controlling the amount of oxygen inspired by a
patient. Many changes, modifications, variations and other uses and
applications of the subject invention will, however, become
apparent to those skilled in the art after considering this
specification and the accompanying drawings, which disclose a
preferred embodiment thereof. For example, the PID controller of
the present invention could in fact be a PI controller or! some
other type of controller having adjustable response
characteristics. Furthermore the feedback control loop could be
integrated into the ventilator 10 or pulse oximeter 16. All such
changes, modifications, variations and other uses and applications
of the invention are deemed to be covered by the claims which
follow.
* * * * *